Citrus tristeza virus (CTV) was purified from bark and leaf tissue of several citrus species. Tissue frozen in dry ice was pulverized and extracted several times with 0.10 M tris-HCl, pH 7.6 for bark or pH 8.4 for leaves. After several polyethylene glycol precipitation steps, concentrated viral suspensions were further purified by centrifugation in CsCl or Cs2SO4 density gradients. Antisera to formaldehyde-treated CTV, or to untreated CTV preparations reacted with purified CTV in microprecipitin tests. However, antiserum produced to untreated CTV reacted with viral antigen degraded in sodium dodecyl sulfate (SDS) in immunodiffusion tests in agar gels containing SDS and sodium azide, but antiserum to formaldehyde-treated CTV did not. Polyacrylamide gel electrophoresis of SDS-degraded CTV showed 1 prominent protein component which had a MW of about 25,000 daltons, and a faster moving, fainter-staining region which probably was a degradation product(s) of CTV protein. Material eluted from the major protein zone reacted strongly with CTV-antiserum in SDS-immunodiffusion tests, while that from the faster-moving region reacted weakly. Some preparations of antiserum reacted weakly with extracts from healthy plants. Antiserum absorbed with healthy citrus tissue preparations was specific for CTV antigen. Citrus tristeza virus antigen was easily detected in extracts from young citrus bark and leaf tissue in SDS-immunodiffusion tests, thus indicating the usefulness of this technique for rapid diagnosis of CTV.